1. Field of the Invention
The present invention relates to a fuel supply system, particularly of the kind for use in a multi-cylinder internal combustion engine and adapted to supply fuel to an intake passage at a place upstream of a branch passage collecting portion.
2. Description of the Prior Art
A fuel supply system adapted to supply fuel to an intake passage at a place upstream of a branch passage collecting portion is well known in the art. A fuel supply system including a carburetor and a so-called single point injection system are of this kind.
An example of a prior art single point fuel injection system is disclosed in Japanese Provisional Patent Publication No. 56-146050 and also shown in FIG. 7. Referring to FIG. 7, designated by the reference numeral 101 is an intake pipe to be connected to a branch collecting portion of an intake manifold, by 102 a throttle valve disposed in an intake passage 103 of the intake pipe 102, and by 104 an electro-magnetic fuel injection valve. The fuel injection valve 104 is supplied with fuel of which pressure is controlled by a regulator 105 so as to be different from induction vacuum by a constant value and varies the quantity of injected fuel with variation of valve opening time. The valve opening time is controlled by a duty ratio of a pulse signal supplied from a control circuit 108 to the fuel injection valve 104. The duty ratio of the pulse signal is determined based upon an intake air quantity signal I.sub.1 from an airflow meter 107 and an engine rpm signal I.sub.2 from a crank angle sensor (not shown).
In general, a single point fuel injection system is encountered by a problem of an unequal distribution of fuel to engine cylinders due to the difficulty in atomization of injected fuel.
To solve this problem, the fuel injection system of FIG. 7 is provided with an additional air supply passage 110 bypassing the throttle valve 102 and reaching the place forward of a nozzle portion 104A of the fuel injection valve 104 so that based upon the pressure differential between the passage portions upstream and downstream of the throttle valve 102 high-speed air streams are introduced to the place forward of the nozzle portion 104A to collide with injected fuel to atomize the same.
However, in the fuel injection system of FIG. 7, since the quantity of additional air introduced to the place forward of the nozzle portion 104A for atomization of fuel varies depending upon the pressure differential between the passage portions upstream and downstream of the throttle valve 102, it tends to be insufficient when the opening degree of the throttle valve becomes large, making it impossible to attain good atomization of fuel and causing some injected fuel droplets to deposit on the inner wall of the intake passage to flow therealong. Unequal or uneven distribution of fuel therefore results to boost fuel consumption and exhaust emissions of hydrocarbons, carbon monoxides and oxides of nitrogens at unacceptable levels.